Balanitis: This is an inflammation of the glans (the rounded head of the penis) that can extend to the penile shaft, often caused by poor hygiene, infections (bacterial, fungal, or viral), or allergic reactions. It's common and can cause swelling, redness, and pain.

Phimosis: A condition where the foreskin cannot be fully retracted over the glans, which can lead to swelling and inflammation of the penile shaft due to trapped moisture and potential infection.
Paraphimosis: A more severe condition than phimosis, where the foreskin is retracted behind the glans and cannot return to its original position, cutting off blood flow and causing swelling.
Cellulitis: A bacterial skin infection that can cause redness, swelling, and pain in the affected area, including the penile shaft.
Urethritis: Inflammation of the urethra, often due to infection, which can cause swelling, discharge, and pain in the penile shaft.

Necrotizing Fasciitis: A severe bacterial infection that destroys tissue under the skin, including fat and the tissue covering the muscles (fascia). It's rare but can be deadly if not treated promptly.
Penile Fracture: A rupture of the tunica albuginea, the fibrous layer surrounding the corpora cavernosa, usually occurring during an erection. It's an emergency that requires immediate medical attention.
Malignancy (e.g., Squamous Cell Carcinoma): Although rare, any unexplained or persistent swelling should be evaluated for the possibility of cancer.

Lymphogranuloma Venereum (LGV): A sexually transmitted infection caused by certain types of the bacteria Chlamydia trachomatis, which can cause swelling, pain, and other symptoms in the genital area.
Fournier's Gangrene: A rare but serious condition characterized by necrotizing fasciitis of the genitalia, which can start with swelling and progress rapidly.
Erythema Multiforme: An immune-mediated disorder that can cause skin lesions and swelling, including on the genitalia, often in response to an infection or medication.